Diurnal and Circannual Variation in Body Temperature: Implications for Heat Illness Protocols

Abstract

As warm-season temperatures continue to rise, the incidence of heat illness is likely to increase. Although preventive protocols currently consider environmental risk factors, there may be reason to emphasize diurnal and seasonal influences. Data supporting seasonal fluctuation in body temperature are abundant in animals but limited in humans. PURPOSE: To examine circannual and diurnal patterns of body temperature in a patient population. METHODS: We analyzed 2,184 men and women admitted to a major hospital in Indiana over 3 years. Demographic, anthropometric, and cardiometabolic variables were collected along with season, month, and time of admission. The National Centers for Environmental Information’s National Climate Report was used to generate month-by-month ambient temperature data; there were clearly defined cold (October through April) and warm (May through September) periods. All patients received oral temperatures. We used t-tests and ANOVAs to detect differences in body temperature by time conditions; we used linear regression to test the effect of chronological variables on body temperature, holding all measured confounders constant. RESULTS: Mean body temperature was 98.16 ± 0.73°F. The warmest period of the day was 6:00 to 10:00pm (98.27°F). The coldest period was 2:00 to 6:00am (98.05°F; p < 0.001). ANOVA revealed differences in body temperature by month (F = 2.525; p = 0.004) and by season (F = 3.656; p = 0.012). The strongest comparison was the cold vs. warm period (T = -3.835; p < 0.001). Patients admitted during the cold period (N = 1,139) had a temperature of 98.10 ± 0.81°F while patients admitted during the warm period (N = 1,045) had a temperature of 98.22 ± 0.63°F (p < 0.001). Lower temperatures were also found among patients ≥ 65 years (p < 0.001) and those with a positive blood alcohol test (p = 0.004). Holding all measured confounders constant, being admitted during the warmer months predicted an elevation in body temperature of 0.13°F (p < 0.001). CONCLUSION: These findings support diurnal and circannual variations in humans. Coaches and athletic trainers may consider this when designing and monitoring practice conditions. Athletes will likely require closer observation along with updated prevention protocols when practicing outdoors to minimize the risk of a heat-related event.